The present disclosure relates to an improved three-way dimming ballast circuit for a gas discharge lamp such as a compact fluorescent lamp (CFL) ballast which is adapted to be used with a conventional three-way dimming Edison screw base.
FIG. 1A shows the well-known Edison screw base for an incandescent bulb having a single light output level. FIG. 1B shows a modification, a three-way dimming Edison base, which was developed for the known three-way incandescent bulb, having two contacts for two different filaments, and a common neutral contact. By the operation of a three-way dimming switch, as illustrated in FIG. 2, the two different filaments, having different power ratings, can be selectively operated separately or together to produce three different lumen outputs, with different corresponding power usage. The three-way dimming switch has four different positions, which provide three different light levels and an off selection.
The conventional dimming lamp base and dimming circuit of FIGS. 1B and 2 have been adapted for use with the energy-efficient compact fluorescent lamp (CFL).
U.S. Pat. No. 7,109,665, incorporated by reference, discloses a three-way dimming CFL ballast that operates at a single high bus voltage, while varying the switching frequency to control lamp current. The ballast accepts three different light level input switch command conditions and an off condition to provide three different light output levels. A closed loop current feedback control maintains a precise switching frequency for the ballast to produce accurate light output levels based on the selected light output switch selection. A reference voltage is provided to the feedback loop based on the selected light output level to drive the switching frequency at the appropriate value. The ballast circuit provides CFL control for three-way dimming and is adapted to be used with the three-way dimming Edison screw base illustrated in FIG. 1B and the switching circuit illustrated in FIG. 2, with a pair of live inputs 51 and 52 being connected to the two live outputs of the dimming switch via the contacts Live1 and Live2 on the lamp base.